This invention relates to a level shifting and filtering circuit, and more particularly, to a level shifting and filtering circuit for developing a D.C. voltage signal substantially free of A.C. voltage and having a voltage level proportional to the difference between the D.C. components of a first and a second voltage, each of which includes D.C. and A.C. components.
In electrical systems, such as high voltage distribution networks, signals are sent over telephone lines, which are commonly called pilot wires, to detect power system faults. It is desirable to continuously monitor the status of the pilot wires. To accomplish such continuous monitoring it may be desirable to determine the difference between the D.C. components of two voltage signals, (1) a first voltage e.sub.a from one pilot wire to a first terminal of a power supply and (2) a second voltage e.sub.b across this power supply appearing between said first terminal and another terminal thereof. The monitoring device typically desires a determination of the D.C. current flowing in the pilot wires which is a voltage proportional to the D.C. pilot wire current flowing through a resistor imposed between e.sub.a and e.sub.b. Because the pilot wires are sourced with current transformers, a large component of A.C. current may be circulating through the pilot wires, causing a large A.C. voltage to be present on the first voltage signal. The power supply is normally an electronic circuit which typically will have some ripple voltage. The monitoring circuit may be susceptible to erroneous operation if the A.C. voltage signal existing in the proportional D.C. voltage difference signal is too high.
The use of differential amplifiers to develop an electrical signal having a D.C. voltage proportional to the difference between a first and a second voltage signal is well known. Also, the use of reactive components to develop filtering networks to smooth or reduce the A.C. voltage signals, such as may exist on pilot wires or the ripple voltage across a power supply, is well known. However, it is desirable to have one circuit for the development of both an electrical signal proportional to the D.C. voltage difference between two voltage signals and also to filter the A.C. voltages existing on the pilot wire system and power supply.
Accordingly, it is an object of the present invention to provide a monitoring circuit that develops an electrical signal substantially free from A.C. voltage and also proportional to a D.C. voltage difference between a first and a second voltage signal.
It is another object of the present invention to provide a monitoring circuit having substantially greater filtering of a first voltage signal relative to the filtering of a second voltage signal.
Still further, it is another object of the present invention to provide a monitoring circuit in which the transfer function and gain of the circuit may be easily adapted to various conditions.
These and other objects of the invention will become apparent to those skilled in the art upon consideration of the following description of the invention.